An Inconvenient Truth: Mounting Evidence Syria Crossed 'Red Line'

A young boy is consoled after his grandparents appear to have been killed in a government strike on the Sunni village of Al Barra.
(Courtesy of FRONTLINE)

There are now three nations who say they have evidence of the use of chemical weapons by the Assad regime in Syria. Israel's senior military intelligence analyst said the regime used chemical weapons "repeatedly" last month and this week France and Britain wrote confidential letters to the United Nations making a similar claim based on what they call "credible evidence."

According to President Obama, the use of chemical weapons is his "red line" when it comes to the conflict in Syria. But yesterday, in response to these claims, the White House said they haven't yet concluded there is enough evidence that Bashar al-Assad has used chemical weapons.

How does the United States ascertain whether chemical weapons have definitively been used? And what kind of evidence would the administration require to reach that red line?